1. Field of the Invention
The present invention relates to an interior optical cable, and more particularly to an interior optical cable comprising one or more tight buffer optical fibers.
2. Description of the Related Art
In general, an optical cable comprises one or more optical fibers. Such a fiber is used as a medium for transmitting an optical signal. It is also a basic component of an optical communication network. However, optical fibers, due to their inherent characteristics, are sensitive to the influence of physical and environmental circumferences. Moreover, optical fibers are damaged by variations in external temperature, physical impacts in pavement, tensile forces and the penetration of moisture. This damage, in turn, causes failures in transmitting an optical signal.
In order to solve the above problems, optical fibers are produced in the shape of an optical cable including a central tension member, a filling member, an outer coating member, etc.
FIG. 1 is a cross-sectional view of a conventional interior optical cable including a central tension member. With reference to FIG. 1, the interior optical cable comprises a plurality of sub-unit cables 120, a central tension member 110, subsidiary tension members 130 surrounding tight buffer optical fibers 121 within the sub-unit cables 120, an outer coating member 150, and a rip cord 140.
The central tension member 110 provides tensile strength to the interior optical cable. It is located at the center of the interior optical cable. The central tension member 110 includes a first member 111 made of fiberglass reinforced plastic (FRP), and a second member 112 made of a polymeric material such as polyvinyl chloride (PVC) or polyolefin (PO) for coating the circumference of the first member 111. In this manner, the interior optical cable is prevented from being damaged by variation in the external temperature.
Each of the sub-unit cables 120 includes a plurality of the tight buffer optical fibers 121, the subsidiary tension member 130 surrounding the tight buffer optical fibers 121, and a coating member 122 made of a polymeric material such as PVC and adapted as an outermost layer of the sub-unit cable 120.
Each of the tight buffer optical fibers 121 includes a core (not shown) adapted as a medium for transmitting an optical signal, a clad layer (not shown) surrounding the core, a coating layer (not shown) surrounding the clad layer, and a fight buffer layer (not shown) formed by extrusion molding so as to surround the outer circumference of the coating layer.
In order to improve the tensile strength of the interior optical cable, the subsidiary tension member 130 is located between the tight buffer optical fibers 121 and the coating member 122 within the sub-unit cable 120.
The outer coating member 150 serves as the outermost layer of the interior optical cable. It is formed by extrusion molding.
Consequently, the central tension member 110 prevents the interior optical cable from being damaged due to shrinkage rate differences between the sub-unit cable 120 and the outer coating member 150, when the interior optical cable is contracted due to variations in the external temperature.
However, conventional interior optical cables include a central tension member that has a reduced flexibility and an increased volume, thus being limited in pavement use. In order to solve the above limitations, an interior optical cable that does not include a central tension member has been proposed.
FIG. 2 is a perspective view of the conventional interior optical cable, as described above, which does not include the central tension member. In FIG. 2, the conventional interior optical cable comprises a plurality of tight buffer optical cables 210, a subsidiary tension member 220 surrounding the tight buffer optical cables 210, and an outer coating member 230 formed by extrusion molding so as to surround the outer circumference of the subsidiary tension member 220.
FIG. 3 is a cross-sectional view of the tight buffer optical fiber 210 shown in FIG. 2. In FIG. 3, the tight buffer optical fiber 210 includes a core 211 adapted as a medium for transmitting an optical signal, a clad layer 212 surrounding the outer circumference of the core 211, a coating layer 213 surrounding the outer circumference of the clad layer 212, and a tight buffer layer 214 surrounding the outer circumference of the coating layer 213. The tight buffer optical fiber 210 includes the tight buffer layer 214 obtained by coating the outer circumference of the coating layer 213 with a polymeric plastic. Thus, the tight buffer optical fiber 210 is generally adapted to the interior optical cable without the central tension member.
However, the structure of such an interior optical cable without the central tension member is damaged by the variation in the external temperature such as a low or high temperature. Consequently, the physical and optical characteristics of the interior optical cable are deteriorated.
In order to solve the above-described problem, (i.e., the damage to the structure of the interior optical cable by low temperatures) the interior optical cable comprises a tight buffer optical fiber. The tight buffer optical fiber includes a tight buffer layer with a multi-layered structure made of different materials, or a clad layer coated in a thickness larger than that of the conventional optical fiber.
For example, a tight buffer optical fiber including two tight buffer layers includes (1) a coating layer formed with a thickness of 250 μm so as to surround the outer circumference of the clad layer, (2) a first tight buffer layer made of an ultraviolet curing agent with a thickness of 300 to 500 μm so as to surround the outer circumference of the coating layer, and (3) a second tight buffer layer made of a plastic such as polyolefin, polyethylene, nylon, etc., so as to surround the outer circumference of the first tight buffer layer.
A general tight buffer optical fiber has the coating layer with a diameter of 250 μm. On the other hand, the tight buffer optical fiber adapted to the interior optical cable without the central tension member has the coating layer with a diameter of more than 250 μm. Thus, it minimizes the shrinkage due to variation in the external temperature, and the damage due to the shrinkage.
However, when the interior optical cable employs a plurality of the tight buffer layers stacked to have a multilayered structure or the optical fibers has a great thickness, it causes a number of problems. For example, an increase in both the complexity of the manufacturing process and the production cost of the optical cable. Further, in a conventional interior optical cable without a central tension member, there is a great shrinkage rate difference, between the outer coating layer and the sub-unit cable installed within the outer coating layer, due to variations in external temperature. Moreover, the difference in shrinkage between the outer coating layer and the sub-unit cable due to external temperature variations is referred to as post-shrinkage. The above post-shrinkage increases lay ratio of the tight buffer optical fiber located in the interior optical cable. Thus, it increases optical loss. Generally, in forming the outer coating layer of the interior optical cable, when the outer coating layer is extruded, the outer coating layer is quenched with cooling water at a temperature of less than 30 degrees. This process causes the polymeric material of the outer coating layer to be maintained in an unstable state.